It turns out to not be too much of a performance issue to allocate
a new frame each time one is produced, so to reduce lock contention
I added a queue where frames are added to and taken from without
locking the frame for the whole update. I'm hoping this will give
more flexibility to frontend implementations, which can simply
skip or repeat frames if needed.
I wanted to make this a bit more modular, so it's easier in theory to
write external crates that can reuse bits, and selectively compile in
bits, such as adding new systems or new cpu implementations
